1. Field of the Invention
The present invention relates to an apparatus for detecting an ignition range for an internal combustion engine, or more in particular to an ignition range detecting apparatus for the internal combustion engine which is advantageous for the control of the air-fuel ratio, exhaust gas recirculation (EGR) or ignition timing.
2. Description of the Prior Art
The importance of an ignition range detector will be described below with reference to a conventional method of the air-fuel ratio control.
FIG. 1 is a graph showing the relation between the mixture gas supplied to the combustion chamber of the internal combustion engine, the composition of the exhaust gas and the fuel consumption rate for NO.sub.X, HC and CO respectively.
If the mixture of the internal combustion engine is controlled to the neighborhood of the stoichiometrical air-fuel ratio (namely, the air-fuel ratio where complete combustion occurs immediately before HC increases by misfire) in the region X in FIG. 1, the obnoxious components including CO and HC of the exhaust gas are minimized, while NO.sub.X is reduced as compared with the neighborhood of the stoichiometric air-fuel ratio generally employed, so that the exhaust gas is purified advantageously. Further, the fuel consumption rate is minimum in the neighborhood of this stoichiometric air-fuel ratio for economic advantage.
It is thus desirable to control the mixture gas to the neighborhood of the stoichiometric air-fuel ratio which is advantageous from the viewpoint of both purification of the exhaust gas and the economy. It is, however, actually difficult to maintain the stoichiometric air-fuel ratio immediately before misfire always in response to the changes of the operating conditions of the internal combustion engine. As a result, an air-fuel mixture considerably richer than the stoichiometric value is used to assure the ignition of the mixture.
In order to solve this problem, means for detecting the stoichiometric air-fuel ratio just before the misfire is required. A conventional air-fuel ratio sensor for detecting the air-fuel ratio of the mixture gas directly uses a metal oxide semiconductor such as zirconium oxide or zirconia. The zirconia air-fuel ratio sensor, however, is capable of detecting only the neighborhood of the stoichiometric air-fuel ratio (14.5 to 15.0 of the air-fuel ratio in FIG. 1), and incapable of detecting the stoichiometric air-fuel ratio.
Let us discuss about means for detecting the stoichiometric air-fuel ratio just before misfire indirectly. In view of the fact that the stoichiometric air-fuel is associated with the complete combustion immediately before misfire as described above, an air-fuel ratio slightly richer than the one detected immediately before misfire (partial combustion) is detected as the stoichiometric air-fuel ratio. It is therefore possible to detect the stoichiometric air-fuel ratio by means of an ignition range detector for detecting the condition (partial combustion) immediately before misfire. The ignition range detector is useful as will be seen from the foregoing description with reference to the air-fuel ratio.
The difference in combusting conditions is shown by the waveform of pressure in the cylinder of an ordinary internal combustion engine. FIG. 2a denotes the complete combustion of the internal combustion engine, FIG. 2b the partial combustion immediately before misfire, and FIG. 2c misfire.
As will be seen FIGS. 2a to 2c, the area S.sub.2 +S.sub.1 formed by the cylinder pressure P and the crank angle along the abscissa increases as the combustion is improved to a complete state of FIG. 2a. Therefore, by determining a value equivalent to this area S.sub.1 +S.sub.2 and comparing it with a predetermined value, it is possible to determine the combustion state. That is, in the state of the partial combustion immediately before misfire as shown in FIG. 2b the area S.sub.1 +S.sub.2 becomes narrow, and in the state of misfire as shown in FIG. 2c the area becomes S.sub.1. Depending on the operating conditions of the internal combustion engine, however, the area representing the combustion state S.sub.1 +S.sub.2 varies for the same complete combustion.
The difference of cylinder pressure waveform according to the load conditions of an ordinary internal combustion engine is shown in FIG. 3. FIG. 3a shows the high-load combustion state of the internal combustion engine and FIG. 3b the light-load combustion state thereof.
As seen from FIGS. 3a and 3b, the area S.sub.1 +S.sub.2 representing the combustion state greatly varies depending on the high or low load for the same complete combustion. In the case of complete combustion under light load of FIG. 3b, for instance, the area S.sub.1 +S.sub.2 is almost equal to the area for the partial combustion immediately before misfire in the case of high load of FIG. 3a. If a predetermined value .alpha. of a fixed level is used for comparison, therefore, the decision on the combustion state becomes erroneous. In order to solve this problem, the area S.sub.1 concerning the misfire under the same condition is considered. Since the area S.sub.1 represents the engine conditions of the internal combustion engine as it is reduced with the decrease of the load, the ratio between the area S.sub.1 +S.sub.2 representing the combustion state and the area S.sub.1 concerning the misfire under the same condition is determined and compared with the predetermined value .alpha.. As a result, as obvious from FIG. 3, it is determined on a fixed standard whether the partial combustion immediately before misfire or complete combustion is involved regardless of the load. In other words, an ignition range can be determined regardless of the load or other operating conditions.
The area representing the combustion state for misfire cannot be actually determined directly. As seen from FIG. 3, however, the area S.sub.1 ' of a predetermined part (such as the part from -.theta.A to -.theta.C in crank angle in FIG. 3) before the explosion/combustion of the mixture gas in compression stroke is proportional to the area S.sub.1 for misfire. The area S.sub.1 ' is determined and multiplied by a predetermined value K.sub.1 thereby to obtain the area S.sub.1.
In the case where a signal representing the crank angle is divided by N and the pressure signal is digitally processed at points of N divisions so that the resulting values are added to obtain the areas S.sub.1 and S.sub.2, the cylinder pressure value P (-.theta.D) for a predetermined crank angle (such as -.theta.D) before explosion/combustion of the mixture gas in compression stroke is determined and multiplied by a predetermined value K.sub.2, thus producing a value S.sub.1 representing the area for misfire.
In the manner mentioned above, the value S.sub.1 representing the area for misfire is obtained.